1. Field of the Invention
The present invention relates to creasing devices, image forming systems, and creasing methods. More specifically, the invention relates to a creasing device that makes a crease (a fold) on a sheet member (hereinafter, “sheet”) delivered from a preceding stage before the sheet is folded in half, an image forming system including the creasing device, an image forming apparatus, and a sheet finisher that processes a sheet delivered from the image forming apparatus, and a creasing method for use by the creasing device or the image forming system.
2. Description of the Related Art
What is called saddle-stitched or center-folded booklet production has been conventionally performed. The saddle-stitched booklet production is performed by saddle stitching a sheet batch, which is a stack of a plurality of sheets delivered from an image forming apparatus, and folding the thus-saddle-stitched sheet batch in the middle of the sheet batch. Folding such a sheet batch containing a plurality of sheets can cause outer side sheets of the sheet batch to be stretched at a fold line by a greater amount than inner side sheets. Image portions at the fold line on outer side sheets can thus be stretched, resulting in damage, such as coming off of toner, to the image portions in some cases. A similar phenomenon can occur when other folds, such as a z-fold or a tri-fold, are performed. A sheet batch can be folded insufficiently depending on the thickness of the sheet batch.
A creasing device, called a creaser, that forms a crease in a sheet batch before the sheet batch undergoes half fold or the like folding operation so that even outer side sheets can be readily folded, thereby preventing coming off of toner has already been known.
An example of such a creasing device is disclosed in Japanese Patent Application Laid-open No. 2008-081258. The creasing device disclosed in Japanese Patent Application Laid-open No. 2008-081258 includes an annular protrusion provided along a perimeter of one roller for forming a crease and an annular concavity created along a perimeter of the other roller so that a pair of the rollers form a crease, having a precise and favorable shape according to a type of the sheet, that extends in a sheet-conveying direction on a sheet when the sheet passes through meshing between the annular protrusion and the annular concavity of the rollers. In the creasing device, the rollers are interchangeable with optimum rollers for a sheet to be creased.
Meanwhile, the creasing device disclosed in Japanese Patent Application Laid-open No. 2008-081258 includes the annular protrusion and the annular concavity provided on the perimeters of the paired rollers and forms a crease extending in a sheet conveying direction by causing the sheet to pass the meshing between the rollers. In this technique, in every sheet to be folded, a crease is formed in a to-be-folded portion from an outward side, which is to become an outer side when the sheet is folded, and then the crease is pushed by a push-out member from an inward side, which is to become an inner side when the sheet is folded, to prevent colorant from coming off the sheet. With this configuration, a folding position is likely to deviate from an intended position because the sheet is pushed out to an outward side. In terms of accuracy of folding position, a crease is preferably formed in a to-be-folded portion on an inward side, which allows accurate positioning of a fold. Put another way, importance has conventionally been placed on preventing coming off of colorant and no particular attention has been paid to accuracy in the positioning of the fold.